The invention herein generally relates to the field of imaging systems for use in the graphic arts industry. More particularly, the invention relates to detection of improperly installed printing plates on an external drum platesetter used to image printing plates.
Printing plates, often made from aluminum sheets or polymer based materials, are manufactured in various sizes. Platesetters, machines used to transfer an electronic image onto a printing plate, must be capable of accommodating different size printing plates.
One technique of mounting a printing plate is to provide a clamp (or clamps) positioned on the outside surface of an external drum platesetter in a fixed location. This fixed position clamp is used to hold one end of the plate, often referred to as the leading edge, onto the external drum. A second clamp, also positioned on the outside surface of the external drum platesetter is moveable, and may be positioned most anywhere on the outside circumference of the drum. The moveable, or xe2x80x9cvariablexe2x80x9d clamp is preset a predetermined distance from the fixed clamp corresponding to the longitudinal length of a particular printing plate to be mounted. The variable clamp is used to hold the end of the printing plate that is opposite from the end being held by the fixed clamp. The end of the plate held by the variable clamp (or clamps) is often referred to as the trailing edge. The variable clamp may be either manually positioned, or automatically positioned via a motor and a controller.
A common failure mode of a platesetter is displacement of one end of the printing plate from under one of the clamps during spin up of the drum. This can occur because of a failure of one of the clamps, or because the variable clamp was set an incorrect distance from the fixed clamp for the size plate being used. A spinning drum having a printing plate clamped at only one end is considered a dangerous condition. In addition to destroying the plate and possibly the platesetter itself, the loose end of the plate can injure an operator or even fly off the drum.
Adding components to the existing system to detect a loose plate increases product cost, decreases reliability, and adds complexity to the system, and hence is undesirable.
The invention herein solves at least the problem of spinning up a drum to full speed, having a plate, or other substrate either partially or completely unsecured onto the drum. The invention herein prevents injury to an operator caused by:
1) a loose printing plate when an operator attempts to stop the drum or otherwise intervene, and
2) precluding a printing plate from flying off the drum.
The invention herein automatically detects a printing plate or other recordable substrate that is not securely fastened to a rotating drum machine, and subsequently stopping rotation of the drum.
The invention herein uses an existing autofocus sub-system in an imaging system to detect an unsecured printing plate mounted to an external drum imaging machine while rotating the drum at a fraction of normal operating speed. The drum is slowly rotated to position the leading edge clamp proximate the incident light beam used for autofocusing. This maneuver places one end of the printing plate such that an autofocus light beam is incident upon (or impinges on) one end of the plate. The autofocus system positions a focusing lens such that optimum focus is obtained. The focus lens is then fixed in place. The drum is slowly rotated at only a fraction of the normal operating speed keeping centrifugal forces low (compared to full speed) thereby preventing the plate from being a danger to an operator and/or the machine itself.
The drum is slowly rotated so at least a portion of the length of the plate is exposed to the autofocus light beam. Preferably, at least one half of the length of the plate is exposed to the autofocusing beam. As the drum is rotated, an autofocus signal is monitored. If the autofocus signal does not deviate from a predetermined value, the plate is determined to be securely mounted, and the drum is spun up to normal operating speed. If the autofocus error signal exceeds a predetermined value, the plate is determined to be unsecurely mounted and the rotation of the drum is immediately stopped.